A medical linear accelerator (LINAC) is a common tool used within oncology community in the fight against cancer. (FIG. 1) These devices deliver targeted radiation to the tumor sight in an accurate and reproducible manner. Routine quality control (QC) is required on the LINAC to ensure that (A) the LINAC is delivering the correct amount of radiation and (B) the LINAC is directing the radiation to the correct location within the patient.
The tests that are used to monitor the amount of radiation given typically involve measuring the radiation output delivered to a device that simulates the patient. These devices, referred to as phantoms, are usually water filled tanks with robotics that position a radiation measuring sensor at various predefined depths within the water. The phantom is typically positioned within the radiation beam at a configuration and distance as shown on FIG. 2.
A typical water phantom is shown on FIG. 3. The water phantom typically consists of a water tank (6) with leveling feet (13) and a water drain valve (12) at its bottom. An arm (10) whose height is controlled by a drive mechanism (11) in the tank controls the height of an arm on which a holder (9) houses a radiation detector (8). A microprocessor is included in the main unit (7).
Taking radiation measurements with a water phantom shown on FIG. 3 requires positioning the radiation detector at very accurate depths within the water. If the radiation detector is at the wrong depth, even by a fraction of a millimeter, the reading will be wrong. This would ultimately adversely affect the patient's treatments. To position the typical existing detector at the correct depth, a two-step process shown on FIG. 4 is used:                (A) The user adjusts the height of the motorized arm until the radiation detector (8) is positioned such that the water surface is at the center of the radiation detector (8). This established a zero point.        (B) the user then adjusts the depth of the detector with the aid of the motorized positioning system to position the radiation detector precisely at depth D.        
Step A is very subjective and difficult to accomplish accurately due mainly to water meniscus and parallax effects. The subjective nature of the process means that users typically double and triple check their work to ensure that no errors have been made. But this extra effort may not result in the true surface and it increases the time required for positioning the chamber.